1. Field of the Invention
The present invention relates to a solid electrolytic capacitor in which a capacitor element having a cathode layer formed on a surface of an anode member having a valve action is covered by resin, and also relates to a method for manufacturing such a solid electrolytic capacitor.
2. Description of Related Art
A conventional solid electrolytic capacitor is disclosed in JP-A-2005-45235. FIG. 4 is a front sectional view of that solid electrolytic capacitor. The solid electrolytic capacitor 1 has a capacitor element 10 in which a surface of an anode member molded of a valve-action metal is covered with a cathode layer formed of a solid electrolytic layer. Around the cathode layer, a cathode draw-out layer 15 is formed. At one face of the anode member, an anode wire 16 is led out of it, and the anode wire 16 is joined, by welding or the like, to an anode terminal 4 formed of a lead frame or the like. To the cathode draw-out layer 15, a cathode terminal 5 formed of a lead frame or the like is joined via an adhesive layer 6 of silver paste or the like.
The capacitor element 10, along with the anode wire 16, is covered by a packaging member 3 formed of hard resin such as epoxy resin. The packaging member 3 is transfer-molded, by being injected in a melted state into a cavity inside a mold in which the capacitor element 10 is placed, so as to mold the capacitor element 10 and then harden. At this time, the shock resulting from resin injection for the transfer molding of the packaging member 3 acts upon the capacitor element 10, and thus the capacitor element 10 receives mechanical stress. The capacitor element 10 also receives mechanical stress resulting from the contraction of the packaging member 3 during its hardening.
Exposing the capacitor element 10 to mechanical stress results in significantly increased leakage current. The capacitor element 10 then needs to be repaired by being subjected to a so-called aging process in which a voltage is applied to the capacitor element 10 at high temperature. If, however, the damage at the time of molding is severe, repair by aging is difficult, leading to diminished yields due to short circuiting and unduly large leakage current.
Even after the solid electrolytic capacitor 1 is finished as an end product, when a user solders the anode and cathode terminals 4 and 5 by reflow soldering or the like, the packaging member 3 expands and contracts abruptly. This causes the capacitor element 10 of the solid electrolytic capacitor 1, which has undergone an aging process, to receive mechanical stress again, leading to the problem of increased leakage current.
To solve these problems, JP-A-H5-136009 discloses a solid electrolytic capacitor in which a protective layer (shock-absorbing layer) is provided between the packaging member 3 and the capacitor element 10. FIG. 5 is a front sectional view of that solid electrolytic capacitor. In this figure, such parts as find their counterparts in FIG. 4 described above are identified by common reference signs.
The capacitor element 10, along with the anode wire 16, is covered by a protective layer 2, and outside the protective layer 2, the packaging member 3 formed of hard resin such as epoxy resin is formed as a thin-layer cover. The protective layer 2 has a smaller linear expansion coefficient than the packaging member 3, and is formed of low-stress silicone resin or the like.
The packaging member 3 is transfer-molded, by being injected in a melted state into a cavity inside a mold in which the capacitor element 10 is placed, so as to mold the capacitor element 10 and then harden. The contraction of the packaging member 3 during its hardening is absorbed by the elasticity of the protective layer 2, and this suppresses the mechanical stress on the capacitor element 10.
Moreover, the shock resulting from resin injection for the transfer molding of the packaging member 3 is absorbed by the soft protective layer 2, and this suppresses the damage to the capacitor element 10. Thus, it is possible to suppress leakage current resulting from mechanical damage to the capacitor element 10.
In addition, when the anode and cathode terminals 4 and 5 are soldered by reflow soldering or the like, the packaging member 3 and the protective layer 2 expand and contract abruptly. At this time, in a similar manner as described above, the expansion and contraction of the packaging member 3 are absorbed by the elasticity of the protective layer 2, and this suppresses the mechanical stress on the capacitor element 10. Furthermore, since the protective layer 2 has a smaller linear expansion coefficient than the packaging member 3, even if the heat of reflow solder conducts to the solid electrolytic capacitor 1, the protective layer 2 expands less than the packaging member 3, and this prevents the packaging member 3 from rupturing.
Inconveniently, however, according to the solid electrolytic capacitor 1 disclosed in JP-A-H5-136009 mentioned above, the silicone resin or the like used for the protective layer 2—a low-stress one which in addition has a smaller linear expansion coefficient than the hard resin such as epoxy resin forming the packaging member 3—is very expensive. This leads to the problem of the solid electrolytic capacitor 1 requiring an increased cost.